LIGO/Virgo/KAGRA S230708t

The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:

We identified the compact binary merger candidate S230708t during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-07-08 05:37:05.120 UTC (GPS
time: 1372829843.120). The candidate was found by the MBTA [1] and
GstLAL [2] analysis pipelines.

S230708t is an event of interest because its false alarm rate, as
estimated by the online analysis, is 4.3e-08 Hz, or about one in 8
months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230708t

The classification of the GW signal, in order of descending
probability, is BBH (97%), Terrestrial (3%), NSBH (<1%), or BNS (<1%).

There was a high rate of noise transients (glitches) in the LIGO Hanford 
detector which may affect the parameters or the significance of 
the candidate.

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object is consistent with a neutron star mass
(HasNS) is <1%. [3] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [3] Both HasNS and HasRemnant consider the
support of several neutron star equations of state. The probability
that either of the binary components lies between 3 and 5 solar masses
(HasMassgap) is <1%.

Three sky maps are available at this time and can be retrieved from
the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by
BAYESTAR [4], distributed via GCN notice about 27 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [4], distributed via GCN notice about 36 seconds after the
candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by
BAYESTAR [4], distributed via GCN notice about 5 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For
the bayestar.multiorder.fits,2 sky map, the 90% credible region is
2292 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 3372 +/- 1101 Mpc (a posteriori mean
+/- standard deviation).

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide
https://emfollow.docs.ligo.org/userguide/.

 [1] Aubin et al. CQG 38, 095004 (2021)
 [2] Tsukada et al. arXiv:2305.06286
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [3] Chatterjee et al. ApJ 896, 54 (2020)
 [4] Singer & Price PRD 93, 024013 (2016)

The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:

We have conducted further offline analysis of the LIGO Hanford Observatory
(H1) and LIGO Livingston Observatory (L1) data around the time of the
compact binary merger (CBC) candidate S230708t (GCN Circular 34194
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 ). 
Parameter estimation has been performed using Bilby [1] and a new sky map,
Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is
available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S230708t

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible
region is 1227 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 3010 +/- 988 Mpc (a posteriori mean
+/- standard deviation).

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide
https://emfollow.docs.ligo.org/userguide/.

[1] Ashton et al. ApJS 241, 27 (2019)